Those skilled in the art know that in traditional tandem axle systems a forward drive assembly is used to distribute rotational power from a driveline to a set of forward wheels and a rear drive assembly is used to distribute rotational power from the driveline to a set of rear wheels. The forward drive assembly comprises a left-handed ring gear and a right-handed pinion and the rear drive assembly has a right-handed ring gear and left-handed pinion. The forward drive assembly also traditionally has a set of helical gears used to transfer half of the rotational power from an inter-axle differential to the forward axle pinion and ring gear.
In one type of tandem axle system described in UK Patent Application No. 2,029,521, a drive unit for a tandem axle vehicle is taught having two differential gear units located within a three-part housing. The drive unit comprises coaxially aligned input and output members and opposed laterally extending axle shafts. The first differential gear unit has a differential casing driven by the input member, a first output gear driving a hollow hypoid pinion and second output gear driving the output member. The second output gear drives the output member via a through shaft which is coaxial with the input and output members. The through shaft extends through the hollow hypoid pinion. The second differential gear unit is offset to the side of the through shaft. The second differential gear unit comprises a differential casing driven by a hypoid gear, which itself is driven by the hypoid pinion. Output gears in the second differential gear unit drive the axle shafts.
Great Britain Patent No. 743,027 teaches a tandem axle system comprising basins 10, 10a and associated hollow axle arms 11, 11a. The hollow axle arms contain live axles to drive the wheels. The front axle comprises a hypoid pinion and a hypoid crown wheel transmitting the drive to a differential gear 14 via a hollow shaft carrying a pinion. A shaft located through the hollow shaft and the pinion transmits drive to the rear axle.
U.S. Pat. No. 1,791,138 provides for a single rear axle drive located in a housing in one embodiment. Power is provided to the rear axle by the transmission shaft x to the solid shaft g. The solid shaft transmits power to the hollow shaft e through hub h and sleeve h1. The ring gear is driven by the pinion, which is mounted on the hollow shaft. The housing provides for a support for bearing k5 and k6.
The above-described prior art design requires distinct parts to be used for the forward and rear drive assemblies. The distinct parts undesirably require manufacturers to produce and maintain a large and expensive parts inventory. Additionally, the distinct parts increase the complexity and expense of tandem axles.
The helical gears are also disadvantageous to known tandem axles. Specifically, the helical gears result in a different axis height between the forward axle output and the rear axle input. The height difference requires different axle pinion angles to be used for the interaxle u-joint driveline angles. When these angles are not the same, or not nearly the same, adverse torsional loading and vibration in the drivetrain assembly may result.
Tandem axle differential housings that do not require helical gears are known to those skilled in the art. For example, U.S. Pat. No. 6,200,240 provides for a forward drive assembly comprising a power divider and a hollow, forward pinion gear. A through shaft is located through the forward pinion gear, although the through shaft does not rotate with the forward pinion gear. The forward pinion gear rotates a main differential and the through shaft rotates a rear pinion gear of the rear differential. The forward pinion gear, the through shaft and the rear pinion gear are taught to be coaxial.
The power divider and the forward pinion gear are rotatably supported within a differential cover by an input bearing and a power divider differential bearing. A bearing cage is attached to the differential cover to support an outer pinion bearing for the forward pinion. A first shim pack, as known to those skilled in the art, is likely required to ensure the proper position of the pinion gear. Additionally, a second shim pack is also required to set the preload for the outer and inner pinion bearings. The forward drive assembly also has an inner pinion bearing for the forward pinion and a left half differential bearing and a right half differential bearing for the main differential.
The above-described design has several disadvantages. For example, a minimum of four bearings are used to rotatably support the input assembly, power divider and pinion gear. Each bearing adds cost and complexity to the design. Furthermore, as mentioned above, some differential carrier housings require a shim pack to position the pinion gear and another shim pack to preload the pinion bearings. Installation of each shim pack leads to increased cost, complexity and assembly time. Additionally, the carrier housing, which includes the bearing cage and a cover, can allow fluid to leak from the differential and/or allow contaminants into the differential.